In the manufacture of semiconductors that are formed using gallium arsenide (GaAs) wafers, device feature dimensions such as integrated circuit implant depth and gate width continue to decrease. To form such features, deposition of refractory metals is typically employed. Conventional processes for depositing refractory metals have the potential of causing damage to GaAs substrates that adversely affects devices being formed thereon that have features having such decreased dimensions. Physical vapor deposition (PVD) has become recognized as a process useful to deposit refractory metals to form sub-micron sized features such as, for example, ohmic and Shottky contacts, on GaAs wafers.
Certain stages of a PVD process, such as the plasma ignition stage, can subject the surface of a GaAs substrate to damage. For example, implanted n-type GaAs surfaces that are exposed directly to the plasma ignition stage in a DC magnetron PVD system can incur damage from impinging secondary electrons, from ions, and from reflected fast neutral atoms that are produced during plasma ignition. The damage caused by a DC magnetron sputter coating PVD process is approximately 10.sup.-3 times that of typical plasma processing techniques such as reactive ion etching (RIE) and electron cyclotron resonance (ECR) etching. Although DC magnetron sputtering produces relatively low damage to GaAs substrates, the performance of some devices such as field effect transistors (FETs) can be affected by damage resulting from DC magnetron PVD. Device parameters affected by the damage are FET gain, breakdown voltage, and transconductance. Typical of such damage is the implantation of ions and neutral atoms, the production of broken bonds or the formation of dangling bonds in the surface microstructure, and the changing of the density and type of surface states.
Unlike RIE and ECR, the damage induced by DC magnetron PVD sputtering is self-limiting. For example, regardless of process time, within the first second of the process 75% of the total damage is completed and after two seconds, 100% of the damage has taken place. Due to the physics of DC magnetron PVD processing, the damage that it produces on GaAs substrates is believed to be localized to a region adjacent the surface (30 .ANG. to 60 .ANG. deep) of the GaAs implanted region. Nonetheless, the damage produced during plasma ignition is sufficient to materially reduce the quality and quantity of devices produced by PVD.
To date, when sputtering refractory ohmic or gate metal using a PVD sputter apparatus, such as a batch processor or a cluster style tool, the placement of a physical shutter in front of the substrate has been needed to protect the substrate such as a GaAs substrate from damaging particles created during plasma ignition. Although a physical shutter is effective to reduce damage during plasma ignition, there are several disadvantages to using a physical or mechanical shutter in a manufacturing environment. For example, since a physical shutter is typically situated above the wafer, it receives a large amount of metal deposit. As it moves in a vacuum, it generates a significant number of particles, typically adding more than 500 particles of 0.5 microns or larger. As gate sizes decrease, high particle density increases the number of defective devices produced and thereby reduces device yield. Further, a physical shutter is prone to mechanical failures and requires preventive maintenance, both of which add to system downtime.
Accordingly, there remains a need to eliminate the requirement for a physical shutter in the PVD of refractory metals onto GaAs substrates while providing damage free deposition.